Ioversol was disclosed as a useful nonionic X-ray contrast agent in U.S. Pat. No. 4,396,598. In the synthesis of Ioversol, amyl acetate is used for the crystallization of N,N'-Bis(2,3-dihydroxypropyl)-5-[N-(2-hydroxyethyl) glycolamido]-2,4,6-triiodoisophthalamide hexaacetate (ester) (hereinafter referred to as "hexaacetate") and wash of the hexaacetate filter cake. After the amyl acetate is used for the crystallization and wash steps, the crude amyl acetate or mother liquor contains several impurities. A few of these impurities are 1,1,2-trichloroethane (hereinafter referred to as "TCE"), bromoethyl acetate (hereinafter referred to as "BEA"), dimethylsulfoxide (hereinafter referred to as "DMSO"), ethylene glycol monoacetate (hereinafter referred to as "EGMA") and ethylene glycol diacetate (hereinafter referred to as a "EGDA").
Until this time, the purification process used to recover amyl acetate for reuse was to distill the amyl acetate mother liquor to separate amyl acetate, TCE, BEA, DMSO, EGMA and EGDA from the tars and non-volatile impurities, and then to fractionate the amyl acetate solution to remove the TCE from the amyl acetate solution as a lower boiling point fraction. Following this purification procedure, the higher boiling point amyl acetate solution still contained other high boiling point substances in small quantities, including BEA, EGMA, EGDA and DMSO. These impurities, particularly BEA, are a problem in the purification process of amyl acetate due to a build-up thereof in the amyl acetate after continuous recycling of the amyl acetate in the industrial setting and due to its corrosiveness.
An alternative method of purification for amyl acetate which would eliminate the build-up of BEA and similar such impurities therein is desired. It is an object of the present invention to meet this need.